The present invention relates in general to paper making machines which manufacture webs of paper and, more particularly, to color changes of the web of paper being manufactured through dye rate control.
Tinting or coloring paper by the use of colorants, referred to herein generically as xe2x80x9cdyes,xe2x80x9d to establish a final product color has been common for many years in the operation of paper making machines 100, see FIG. 1. The actual sheet or web color is determined by measuring the reflectance spectrum of the web as it travels through the production process. For example, measurement may be performed by an online spectrometer 101 that measures the reflectance spectrum of the web with measurements normally taking place near the end of the process at a point where the product is nearly complete 102. Dye additions can be made at different stages of the paper making process to achieve a desired color shade. To make a very deep shade of color, dye can be added to a blend chest 104 as part of the base load to allow for a higher concentration of dye on the paper fibers. This is referred to as base loading. Dyes can also be added almost anywhere before the inlet to the fan pump 106 for fine adjustments or compensating color disturbances. This is referred to as color trimming. While the paper is forming on a Fourdrinier wire 108, colorants can be applied on a felt side 108A and/or a wire side 108B of the paper. Colorants can also be added at a size press 110 or in coating materials. The distances between the dye addition points and the color sensor vary dependent upon the points of dye addition. The delay times are also dependent upon characteristics of the device used to add the dye with times commonly being on the order of up to several minutes, several magnitudes greater than the process time constant.
The paper making machine continues to operate while color changes are being made so that the web of sheet material produced during a color change does not meet the specifications for either the original color paper being made or the new color paper to which the process is being changed. This out-of-spec web, referred to as color broke, is recycled back to the early stages of production. Since color broke and its recycling are very costly, reductions in color change times will reduce, possibly substantially reduce, production costs of colored papers.
Automatic control is widely used during normal production to maintain desired color targets. Multiple dyes with complementing characteristics are regulated under automatic control to achieve final product color targets within specification limits. During product color or shade changes an operator can manually make changes to each dye flow actuator until the new product specifications are met. Because of the long delay from the dye addition to the color sensor and a large time lag due to recirculation of the basis product and water 112 at the initial forming section of the paper making machine, color changes can take quite a long time to complete. Color change times of 40 minutes to over 60 minutes are very common when performed manually by production operators.
A method and apparatus for controlling the spectral reflectance of a material such as a paper web is disclosed in U.S. Pat. No. 6,052,194. In the ""194 patent, a comparison is performed between target reflectance values and measured reflectance values in numerous wavelength bands spanning the visible spectrum. The comparison quantifies errors that are modified by a nonlinear operation, such as squaring, and then added together. The resulting sum is minimized by individually adjusting the application of a number of colorants to the web process. The number of wavelength bands is made greater than the number of colorants to avoid metameric effects.
While the ""194 patent illustrates one color change technique, other color change arrangements are needed to provide alternatives and to advance the state of the art. Preferably, such an arrangement would not only reduce the time required to make color changes but also enable improved determinations of dye rates required for given color targets of the paper to be produced by a paper making machine.
The present invention currently meets this need by providing methods and apparatus for making improved web color changes in a paper making machine. In accordance with the present invention, when a web color change is to be made, at least one current dye rate (normally a plurality of dye rates) is determined and a nominal dye rate (or a corresponding plurality of nominal dye rates) representative of the new web color is retrieved from storage. The difference between the current dye rate and the retrieved nominal dye rate, xcex94dyerate, is used in a dye rate trajectory or change response defined by the equation:       ChgDyeRate    ⁡          (      s      )        =            {                        [                                                    τ                1                                            τ                2                                      -                                          (                                                                            τ                      1                                                              τ                      2                                                        -                  1                                )                            ⁢                                                e                                                            -                                              T                        d2                                                              ⁢                    s                                                                                                              τ                      2                                        ⁢                    s                                    +                  1                                                              ]                ⁢                              e                                          -                                  T                  d1                                            ⁢              s                                ⁡                      (                          Δ              ⁢                              xe2x80x83                            ⁢              dyerate                        )                              }        ⁢          u      ⁡              (        s        )            
to change the dye rate thereby changing the web color from a first color to a second color. Improved determinations of color targets or nominal dye rates are made by accumulating historical data representative of production dye rates from past process production for corresponding product colors and combining the historical data with current dye rate data to arrive at new nominal dye rates for colors of web to be produced by the paper making machine. A retrieved nominal dye rate and the current or active dye rate are combined using a filter factor alpha (xcex1). The filter factor xcex1, which is greater than or equal to zero and less than or equal to one (0xe2x89xa6xcex1xe2x89xa61), determines the fraction of the historical dye rate which is combined with the remaining fraction of the active dye rate to determine the new nominal dye rate that is stored as the nominal dye rate for the given product color as shown in the equation:
NewNominalDyerate=(xcex1)SavedNominalDyerate+(1xe2x88x92xcex1)ActiveDyerate.
The active dye rate is preferably taken near the beginning of a run of a given color; however, it can be taken at any point in the run and can be an accumulation of data which is averaged to arrive at the active dye rate used for the determination of the new nominal dye rate to be stored for a given web color.
The invention of the present application will be better understood from the following description, the accompanying drawings and the appended claims.